Narrated Abu Masud:

The Prophet pointed with his hand towards Yemen and said twice, "Faith is there," and then pointed towards the East, and said, "Verily, sternness and mercilessness are the qualities of those who are busy with their camels and pay no attention to their religion, where the two sides of the head of Satan will appear," namely, the tribes of Rabl'a and Muqar.

حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، عَنْ إِسْمَاعِيلَ، عَنْ قَيْسٍ، عَنْ أَبِي مَسْعُودٍ، قَالَ وَأَشَارَ النَّبِيُّ ﷺ بِيَدِهِ نَحْوَ الْيَمَنِ
" الإِيمَانُ هَا هُنَا ـ مَرَّتَيْنِ ـ أَلاَ وَإِنَّ الْقَسْوَةَ وَغِلَظَ الْقُلُوبِ فِي الْفَدَّادِينَ حَيْثُ يَطْلُعُ قَرْنَا الشَّيْطَانِ رَبِيعَةَ وَمُضَرَ ".